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M31 and Messier's catalogue

If you have the chance to live outside a city or at least if you can get out of it easily, you may have already noticed that there is some stuff going on up there. The moon is obviously the easiest object to observe when the sky is clear and, obviously, when it’s not the 'new Moon'.

But there is so many other things to look at. The Milky Way (our galaxy) is a good example: as our solar system is included in it, it is always here. But depending on light pollution and where you live, you might see it differently.

I won't get into details about the Milky Way, but I want to talk about Andromeda Galaxy (M31), the little object pointed in red on the picture below:

Andromeda Galaxy is also called M31, why?

Because of the French comet hunter Charles Messier: this guy was observing comets during the 18th century and, guess what, at that time, he didn't have the Hubble telescope.

The only thing he had was a refracting telescope with a focal lenght of 100mm (it means it can look at objects seemingly twice as small as a human can). It might not come as a surprise if I tell you he often mistook comets with other objects such as galaxies, clusters or nebulas.

Charles decided to create a catalogue in which he would bookmark every object he saw to be sure he wouldn’t mistake it with a comet the next time... and each one of those objects’ name would start with the letter 'M'.

So M31 was the 31st object Charles Messier filed in his catalogue!

What is M31 ?

M31 is a spiral galaxy situated about 2,5 million light-years from Earth.

That's a lot...

But it is nothing....

It is 'only' the closest galaxy...

... in our local group.

Anyway, it is big and massive, and it contains about 1 000 000 000 000 stars!

How can we see it ?

To evaluate the brightness of a celestial object, we use the 'apparent magnitude' scale. This scale uses a logarithmic inverse relation to the brigthness of the object: the brighter it shines, the lowest is its apparent magnitude.

Let's look at an example:

  • The apparent magnitude of the Sun is -26

  • The apparent magnitude of the full Moon is -13

  • But is the Sun twice as bright as the Moon? No, of course, because of the 'logarithmic thing'*, it is about 150 000 times brighter!​

Apparent magnitude of M31 is 3.4, which is 100 000 000 000 times less bright than the Sun and 6 000 000 times less bright than the Moon!!!

* I am not mathematician

To sum up, M31 is not super bright...

Still, you can observe it with the naked eye in a very dark place... but you will only be able to see the brightest part of it: the center. The 'arms' are huge, and if you could see M31 in whole, you would see something apparently 6 times larger than the Moon!

To find it, you first need your eyes to adpat to the darkness (it takes about 15 minutes).

Visible in the northern sky, search for the "W" shape of the Cassiopeia constellation. Then, follow the left section for about 4 times its length, and you will distinguish M31. You might see it better if you don't look at it directly but slightly around it... or if you use binoculars!

The picture

Here is a picture taken with a 270mm F6.3 lense:

This picture was taken in Guérande, France, and is the result of the combination of 28 images of 15 seconds each (7 minutes exposure) taken with a Canon 70D (1600 ISO) and a Tamron 18-270mm f/3.5-6.3.

As objects in the sky are moving due to the rotation of the Earth, I used the equatorial mount of a telescope.

I also used a software to enhance details and crop it.

To conclude

  • it's a galaxy far, far away... ;) (but one of the closest)

  • it's bright

  • it's huge

  • it's beautiful

  • and it maybe contains other solar systems (and planets?) similar to ours !

Don't hesitate to look for info and other pictures of M31, you will find many on astrophotographer websites. Some of those pictures are mindblowing! And of course there is the NASA website!

Hope you will try to look for it next time our night sky is clear enough!

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